This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
Transmission and reception from a node, e.g., a radio terminal like a User Equipment (UE) in a cellular system such as LTE, can be multiplexed in the frequency domain or in the time domain (or combinations thereof). In Frequency Division Duplex (FDD), downlink (DL) and uplink (UL) transmission take place in different, sufficiently separated, frequency bands. In Time Division Duplex (TDD), DL and UL transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired frequency spectrum, whereas FDD generally requires paired frequency spectrum.
Typically, a transmitted signal in a radio communication system is organized in some form of frame structure, or frame configuration. For example, LTE generally uses ten equally sized subframes 0-9 of length 1 ms per radio frame. In case of TDD, there is generally only a single carrier frequency, and UL and DL transmissions are separated in time. Because the same carrier frequency is used for uplink and downlink transmission, both the base station (BS) and the UEs need to switch from transmission to reception and vice versa. An important aspect of a TDD system is to provide a sufficiently large guard time where neither DL nor UL transmissions occur in order to avoid interference between UL and DL transmissions. For LTE, special subframes (e.g., subframe #1 and, in some cases, subframe #6) provide this guard time. A TDD special subframe is generally split into three parts: a downlink part (DwPTS), a guard period (GP), and an uplink part (UpPTS). The remaining subframes are either allocated to UL or DL transmission.
TDD allows for different asymmetries in terms of the amount of resources allocated for UL and DL transmission, respectively, by means of different DL/UL configurations. In LTE, there are seven different configurations. Generally speaking, to avoid significant interference between DL and UL transmissions between different radio cells, neighboring radio cells should have the same DL/UL configuration. Otherwise, UL transmission in one radio cell may interfere with DL transmission in the neighboring radio cell (and vice versa). As a result, the DL/UL asymmetry generally does not vary between radio cells. The DL/UL asymmetry configuration is signaled, i.e., communicated, as part of the system information and can remain fixed for a long time.
Consequently, the TDD networks generally use a fixed frame configuration where some subframes are UL subframes and some are DL subframes. This may prevent or at least limit the flexibility to adopt the UL and/or DL resource asymmetry to varying radio traffic situations.
In future networks, it is envisioned that we will see more and more localized traffic, where most of the users will be in hotspots, or in indoor areas, or in residential areas. These users will be located in clusters and will produce different UL and DL traffic at different time. This essentially means that a dynamic feature to adjust the UL and DL resources to instantaneous (or near instantaneous) traffic variations would be required in future local area cells.
In the current networks, UL/DL configuration is semi-statically configured, thus it may not match the instantaneous traffic situation. This will result in inefficient resource utilization in both UL and DL, especially in cells with a small number of users. In order to provide a more flexible TDD configuration, so-called Dynamic TDD (also sometimes referred to as Flexible TDD) has therefore been introduced. Dynamic TDD configures the TDD UL/DL asymmetry to current traffic situation in order to optimize user experience. For a better understanding of the Dynamic TDD subframe configurations, FIG. 1 illustrates an example Dynamic TDD subframe configuration.
In the illustrated configuration, Dynamic TDD provides an ability of configuring some subframes to be “flexible” subframes, for example, subframes 3, 4, 8, and 9. These flexible subframes can be configured dynamically and flexibly as either for UL transmission or for DL transmission. The subframes being configured as either for UL transmission or DL transmission reply on e.g. the radio traffic situation in a cell. Accordingly, it is expected that Dynamic TDD can achieve promising performance improvements in TDD systems when there is a potential load imbalance between UL and DL. Besides, Dynamic TDD approach can also be utilized to reduce network energy consumption. It is expected that dynamic UL/DL allocation (hence referred in this section “Dynamic TDD”) should provide a good match of allocated resources to instantaneous traffic.
Further, in Layer one (L1) controlled dynamic TDD, whether the flexible subframe is a downlink or an uplink subframe is decided by the eNB and the UE will follow the uplink and downlink grant or some indicators from the eNB to judge the subframe is a downlink or an uplink subframe. If the eNB schedules the UE in the flexible subframe as uplink, then the UE will transmit on the subframe as uplink. Similarly, if the eNB schedules the UE in the flexible subframe as downlink, the UE will receive the downlink signal in the flexible subframe. The uplink scheduling grant could be detected by the UE in the fixed downlink subframe before the flexible subframes. The downlink scheduling grant could be detected in the scheduled flexible subframes. When the UE is not scheduled in the flexible subframes, then UE will treat the flexible subframe as downlink subframe.
In the current 3GPP protocols, when a PUSCH is scheduled, the Uplink Control Information (UCI), if any, will be multiplexed in the PUSCH. The ACK/NACK bits for the downlink transport blocks are one kind of the UCI. According to 3GPP Technical Specification 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, v.11.1.0),                For the case that the UE is transmitting on PUSCH and the PUSCH transmission is adjusted based on a detected Physical Downlink Control Channel (PDCCH) with DCI format 0/4 or with uplink grant intended for the UE and TDD UL-DL configurations 1-6, if VDAIUL≠(UDAI+NSPS−1) mod 4+1, the UE detects that at least one downlink assignment has been missed and the UE shall generate NACK for all codewords where Nbundled is determined by the UE as Nbundled=VDAIUL+2. If the UE does not detect any downlink assignment missing, Nbundled is determined by the UE as Nbundled=VDAIUL. The UE shall not transmit HARQ-ACK if UDAI+NSPS=0 and VDAIUL=4; and        For the case that the UE is transmitting on PUSCH, and the PUSCH transmission is not based on a detected PDCCH with DCI format 0/4 or with uplink grant intended for the UE and TDD UL-DL configurations 1-6, if UDAI>0 and VDAIDL≠(UDAI−1)mod 4+1, the UE detects that at least one downlink assignment has been missed and the UE shall generate NACK for all codewords. The UE determines Nbundled=(UDAI+NSPS) as the number of assigned subframes. The UE shall not transmit HARQ-ACK if UDAI+NSPS=0, wherein VDAIUL is the value of the Downlink Assignment Index (DAI) in DCI format 0/4 and UDAI is the total number of PDCCH(s) with assigned PDSCH transmission(s) and PDCCH indicating downlink SPS release detected by the UE within the subframe(s) n−k in serving cell, where kεK according to table 10.1-1 in 3GPP TS 36.213, which is reproduced as below:        
TABLE 10.1-1Downlink association set index K: {k0, k1, . . . kM−1} for TDDUL-DLSubframe nConfiguration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7, 4, 6————8, 7, 4, 6——3——7, 6, 116, 55, 4—————4——12, 8, 7, 116, 5, 4, 7——————5——13, 12, 9, 8,———————7, 5, 4, 11, 66——775——77—NSPS, which can be zero or one, as the number of PDSCH transmissions without a corresponding PDCCH within the subframe(s) n−k, where kεK for UL-DL configuration 2 or the UL-DL configuration with the most downlink subframes. Nbundled is used to select the scramble code sequence for the HARQ bits (possibly subject to necessary coding) in PUSCH.
In L1 controlled dynamic TDD (referring to R1-130558 “Signalling support for dynamic TDD”, Ericsson, ST-Ericsson), the UE will adjust its scheduling timing for UL and DL based on two reference TDD configurations respectively. The UE will schedule UL transmission based on a reference UL TDD configuration and schedule DL transmissions based on a reference DL TDD configurations. One example is to schedule UL transmissions using TDD configuration 0 and to schedule DL transmissions using TDD configuration 1. In this case, subframe #4 and #9 are used as flexible subframes, which can be used as for either UL or DL.
The benefits with the L1 controlled dynamic TDD are in that it provides fully dynamic control giving the largest performance benefits. It also ensures that control signaling, other than DL scheduling, will not experience cross link interference. It has a natural way of handling HARQ continuity between switches. It also has minimum signaling overhead since the direction is controlled implicitly by the scheduling, which is needed for each transmission anyway.
However, in the L1 controlled Dynamic TDD, the uplink scheduling mechanism is following the scheduling mechanism of the UL-DL configuration 0. Then the two bits for DAI in DCI format 0/4 in UL-DL configuration 1-6 are always treated as UL-DL index in L1 controlled dynamic TDD. Due to this, there will be no more bits for the eNB to inform the UE about the parameter of DAI. Without knowledge of the DAI, the UE cannot determine the value of parameter Nbundled that will be used for scrambling and bundling the HARQ bits on PUSCH.